For protecting the environment, it is required, worldwide, to suppress the amount of carbon dioxide (CO2) emitted from the internal combustion engines such as automotive engines and to suppress the amount of nitrogen oxides (NOx) emitted.
A lean-burn internal combustion engine such as a diesel engine has a feature of emitting CO2 in small amounts owing to its high thermal efficiency accompanied, however, by the defects of emitting NOx in relatively large amounts and, further, producing particulate matter.
It is, therefore, important to decrease both the NOx and the particulate matter in the exhaust gas emitted from the lean-burn internal combustion engines such as diesel engines, and development and improvement have continued concerning the technology for burning fuels, concerning the technology for reforming fuels and concerning the technology for after-treatment.
To cope with these problems, the present applicant has proposed, in Japanese Unexamined Patent Publication (Kokai) No. 2001-271634, an exhaust gas purification system (DPNR) of the type of continuous regeneration which purifies both the NOx and the particulate matter relying on a combination of controlling the air-fuel ratio in the combustion system with a particular catalyst.
According to this system, a lean-burn condition of a high air-fuel ratio (lean) is established for the steady operation, an instantaneous burning condition of a low air-fuel ratio (rich) is established intermittently, the particulate matter is purified, by burning, by utilizing active oxygen generated by a particular catalyst accompanying a variation in the composition of the exhaust gas and, at the same, the NOx is purified by reduction.
The above system essentially requires the development of a particulate matter-oxidizing material which is capable of exhibiting a particulate matter-oxidizing action from a low temperature (not higher than about 300° C.) so that the particulate matter will not be excessively accumulated on the filter under practical operating conditions, and of which the action for purifying NOx by reduction will not be impaired by the oxidizing action.
Ceria has been known for its function as an oxidizing catalyst. For example, Japanese Unexamined Patent Publication (Kokai) No. 10-151348 discloses a particulate matter-oxidizing catalyst using at least either a cerium oxide or a zirconium oxide as a substrate.
There has further been known that a cerium-zirconium composite oxide has an oxygen storage capability (OSC); i.e., Ce atoms contained therein are capable of changing the valency between a valency of 3 and a valency of 4, such as changing the valency from 3 to 4 in an oxidizing atmosphere where O2 is contained in relatively large amounts to absorb oxygen, and changing the valency from 4 to 3 in a reducing atmosphere where CO and HC are contained in relatively large amounts to release oxygen.
Prior arts of the cerium-zirconium composite oxides have been taught in Japanese Unexamined Patent Publications (Kokai) Nos. 2000-169148 and 10-212122, and it has been described that the optimum cerium-zirconium composite oxides which are the OSC materials, usually, contain cerium in an amount as large as about 50 mol %.
It is an object of the present invention to provide a particulate matter-oxidizing material adapted to the system proposed above by the present applicant by utilizing, as a particulate matter-oxidizing material, a cerium-zirconium composite oxide of a composition different from the cerium-zirconium composite oxides proposed by the above prior arts.
Concretely speaking, it is an object of this invention to provide a particulate matter-oxidizing material which enhances the rate of oxidizing the particulate matter at low temperatures (not higher than about 300° C.) which is necessary in the exhaust gas purification system proposed by the present applicant and in other exhaust gas purification systems, and of which the action for purifying NOx by reduction at high temperatures (not lower than about 500° C.) will not be impaired by the oxidizing action.